As one of wireless communication methods according to a related art, there is data transmission based on random access. For example, in the mobile communication system, when uplink data is generated, a mobile station performs random access using a physical random access channel (PRACH), and requests a base station to permit uplink transmission. In the random access, when a plurality of mobile stations transmit the same preamble ID through the same PRACH, contention occurs. When contention occurs, the base station has a problem in detecting the PRACH, and it is finally difficult to detect whether or not the mobile station has performed the random access. Hereinafter, this random access is referred to as “contention-based random access.”
Here, an operation of the contention-based random access according to a related art will be briefly described. For example, when uplink data is generated, the mobile station starts random access, and transmits a randomly selected preamble ID to the base station through Msg1 that is transmitted through the PRACH. Here, a plurality of mobile stations are assumed to transmit the same preamble ID through the same PRACH, and in this case, contention occurs. Then, the base station transmits a response to Msg1 through Msg2 together with, for example, a synchronisation signal or transmission grant for uplink communication. The mobile station acquires an uplink synchronisation timing through Msg2. Then, the mobile station transmits its own identifier through Msg3. Thereafter, the base station resolves the contention through Msg4. In other words, when an identifier of a mobile station is detected, Msg4 is transmitted to the corresponding mobile station to notify of the fact that the mobile station has won the contention. Then, after exchanging Msg1 to Msg4, the mobile station performs transmission and reception of data with the base station.
Besides the contention-based random access, as a random access, there is contention-free random access. When downlink data directed to a certain mobile station is generated, the base station performs the contention-free random access to adjust an uplink synchronisation timing in order to obtain uplink synchronisation when uplink synchronisation with the mobile station is made.
Specifically, first of all, the base station assigns a dedicated preamble ID to the certain mobile station through Msg0. Then, the mobile station starts the random access using the assigned dedicated preamble ID through Msg1 which is transmitted through the PRACH. Then, the base station transmits a response to Msg1 through Msg2 together with, for example, a synchronisation signal or transmission grant for uplink communication. The mobile station acquires an uplink synchronisation timing through Msg2. Then, after exchanging Msg1 and Msg2, the base station starts transmission and reception of data with the mobile station. Further, Msg1 is specified to be transmitted through a most recent PRACH of a subframe in which Msg0 has been received.    Non-Patent Document 1: 3GPP T536.300, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN)”, V10.0.0, Release 10, June 2010.    Non-Patent Document 2: 3GPP T536.321, “Medium Access Control (MAC) protocol specification”, V9.3.0, Release 9, June 2010.
However, in the contention-free random access, there may be a lack of dedicated preamble IDs used in Msg0. For example, it happens when many mobile stations are executing the handover in a cell of a base station. In the handover, a dedicated preamble ID is assigned through Msg0, the random access is performed in a target base station, and an uplink synchronisation timing is fastly obtained. For this reason, when many mobile stations are executing the handover, many dedicated preamble IDs are used. In this case, since it is difficult for the base station to trigger the mobile station to perform the contention-free random access due to a lack of dedicated preamble IDs, a null preamble ID (specifically, “000000”) is transmitted through Msg0. For this reason, thereafter, the mobile station that has received Msg0 performs the “contention-based random access (Msg1 to Msg4)” rather than the contention-free random access (Msg1 and Msg2) using the dedicated preamble ID.
In other words, when there is a lack of dedicated preamble IDs in the base station, there is a problem in that a processing time increases by a time required to transmit and receive Msg3 and Msg4 compared to the contention-free random access method.